It was conventionally a common practice to mix and knead the rubber raw material including, for example, a natural rubber and a synthetic rubber with a predetermined distribution rate involving carbon black, particles sulphur and so on in a rubber kneading machine called a Banbury mixer in the rubber industry so as to produce a variety of rubber products. For example, it is well known that a kneaded rubber for rubber tires is required to have uniform plasticity and dispersibility of the additives therein to enhance the quality of the individual finished tire.
In order to obtain such a kneaded rubber with excellent tire quality, a control method of the rubber kneader has generally been carried out in such a manner that a certain volume of the rubber raw material and the additives is kneaded on trial on the basis of a distribution table so as to seek optimum standard values of kneading period and temperature and an operator thereafter controls the rubber kneader in accordance with the fore-going optimum standard values. The plasticity of the kneaded rubber and the dispersibility of the additives did not have respective desired values causing great irregularity even if the rubber raw material and the additives are kneaded and mixed on the basis of the trial operation by the reasons of irregularity of a distribution rate between the rubber raw material and the additives, abrasion of the kneading rotor, and fluctuation of the operational conditions such as rotational speed of the rotor and the temperature of the machine. As a result of this, the production of intermediate tire products such as tire treads and rubberized cord fabrics with the kneaded rubber with such a great irregularity results in uneveness in rubber weight per unit volume of the intermediate tire product, thereby causing unbalance in weight to the finished tire and thus decreasing steer-ability of vehicles. On the other hand, the irregularity of the plasticity of the kneaded rubber requires tedious arrangement of extruding dies and thus deteriorates operational efficiency by the reasons that a tire tread is extruded through the die which is so designed in its shape as to bear in mind a specified plasticity. The great irregularity of the dispersibility also deteriorates abrasion and fatique rates of the tire, resulting in decreasing longevity of the tire.
FIG. 1 graphically shows the plasticity of the kneaded rubber and the electric energy spent during the mixing and kneading operation of the same kind of rubber raw material in a conventional manner. As will be seen from FIG. 1, the plasticity is designated to have great irregularity in spite of almost constant electric energy spent by the rubber kneader. The fact that the electric energy has relatively small irregularity compared with the plasticity is due to the reason that the operator manages to control the rubber kneader for obtaining a desired or predetermined plasticity of the kneaded rubber relying on his experience and sense while watching the development of conditions such as temperatures and mixing time of the kneaded rubber. It is generally well known in the art that the dispersibility of the carbon black particles in the kneaded rubber becomes more preferable by making the kneading time longer while being brought about irregularity depending upon the plasticity of the rubber raw material by making the kneading time shorter, on the assumption that the revolutions of the kneading rotor of the kneader is controlled almost constant. According to the conventional manner, the dispersibility of the carbon black particles is therefore not always satisfactory even if the rubber raw material is controlled to have a desired plasticity.